The present invention generally relates to an electrophotographic copying method and, more particularly, to a dry electrophotographic copying method for electrically transferring a powder image from an intermediate photoconductive support surface onto a sheet of final support material.
In practicing an electrophotographic copying method of the system referred to above, a two-component developing material has heretofore been employed for developing an electrostatic latent image on the intermediate photoconductive support surface in the form of an electrical potential pattern to produce the powder image corresponding to a pattern of light and shadow to be reproduced. The two-component developing material is comprised of a mixture of toner particles, such as particles of a synthetic resin coloring agent, with carrier beads such as powdery iron or glass beads. When in use, the toner and the carrier are stirred to allow the toner particles to be triboelectrically charged to a polarity opposite to that of the electric potential pattern on the photoconductive support surface, and then applied onto the photoconductive support surface to develop the electrostatic latent image into the toner image.
In the mixture of carrier and toner heretofore used for the development of the electrostatic latent image, the carrier is the component which is not consumed and is recovered for reuse whereas the toner is consumed. Accordingly, for the purpose of achievement of the reproduction of a reasonably acceptable image on the sheet of final support material such as a copying paper, the toner must from time to time be replenished into a developer tank to maintain the proper ratio in the mixture of the toner to the carrier throughout cycles of copying operation. Therefore, the practice of the conventional electrophotographic copying method using the toner-carrier mixture requires the employment of a complicated toner replenishing device. In addition, the replacement of the carrier with a fresh mass of similar carrier particles at regular intervals is also required since the carrier particles tend to be deteriorated, as they are used for a prolonged period of time, to such an extent that the quality of the reproduced image will adversely affected.
Another electrophotographic copying method using a one-component developing material is also well known to those skilled in the art. This one-component developing material is generally employed in the form of a mass of magnetic toner particles each being constituted by a synthetic resin block containing magnetic particles, uniformly dispersed therein, and coated with an electroconductive material such as carbon black. The development of the electrostatic latent image into the toner image according to this method is performed by way of a magnetic brush development technique as is the case with the toner image development using the two-component developing material. Whereas in the toner image development using the two-component developing material, the electrostatic attractive force acting between the toner, which has acquired an electrical charge as a result of frictional electricity, and the electrical charge of the latent image on the photoconductive support surface plays a major role in transferring toner particles onto the photoconductive support surface to form the toner image thereon, a similar transfer in the toner image development using the one-component developing material takes place by the combined effect of a force of electrostatic attraction, exerted between the electric charge of the latent image on the photoconductive support surface and the charge which has been injected, in a polarity opposite to that of the latent image on the photoconductive support surface, through an electroconductive sleeve or shell into the magnetic toner particles as the latter had approached the latent image on the photoconductive support surface, the value of the electric charge so injected corresponding to that of the latent image, and a force of magnetic attraction exerted by a magnet positioned internally of the sleeve or shell for magnetically retaining the magnetic toner particles on the sheeve or shell.
The toner image development using the one-component developing material substantially eliminates such disadvantages inherent in the toner image development using the two-component developing material as resulting from the inclusion of the carrier which forms the unconsumable part of the two-component developing material, but has some disadvantages, for example, the lack of a high fidelity reproduction in gradation, the difficulty in fixing and the inability of use with an ordinary plain copying paper because of the difficulty involved in transferring the toner image from the photoconductive support surface of such plain copying paper by the use of a corona discharge technique. These disadvantages are considered as originating from the fact that the one-component developing material, i.e., the magnetic toner, is required to have a relatively low resistance to facilitate the charge injection from the photoconductive support surface to the magnetic toner through the sleeve or shell during the application of magnetic toner particles onto the electrostatic latent image on the photoconductive support surface. Because of the required use of the one-component developing material of relatively low electric resistance, the toner image development using the one-component developing material is likely to involve the instability of transfer of the toner image from the photoconductive support surface to the sheet of final support material which would result in the insufficient transfer of the toner image onto the sheet of final support material to every detail and/or adherence of magnetic toner particles to non-image areas, i.e., background deposition of the magnetic toner particles. The consequence is that the image reproduced on the sheet of final support material after the toner image transferred onto the sheet of final support material has been fixed will be blurred and/or foggy.
A developing material which substantially eliminates the above described disadvantages and inconveniences inherent in any of the two-component developing material and the one-component developing material is disclosed in, for example, the Japanese Laid-open Patent Publication No. 52-65443, laid open to public inspection on May 30, 1970, and the copending U.S. patent application Ser. No. 863,616, filed on Dec. 23, 1977 and assigned to the same assignee of the present invention.
The process disclosed in the above mentioned publication is a magnetic brush development method wherein the developing material is magnetically attracted onto the sleeve or shell by the action of a magnet, housed inside the sleeve or shell, to form a magnetic brush which subsequently contacts the electrostatic latent image on the photoconductive support surface to develop such latent image into the powder image. The developing material used in this magnetic brush development process and disclosed in the above mentioned publication is comprised of a mixture of a toner of low volume resistivity, for example, 10.sup.5 .OMEGA..multidot.cm, and a toner of high volume resistivity, and at least one of the both toners is a magnetic toner. The toner image development using the mixture of these toners of low and high volume resistivities is carried out by triboelectrically charging both in opposite polarities to each other during the supplying of the toner mixture from a hopper onto the sleeve or shell and/or during the transport of the toner mixture through and by means of the sleeve or shell towards the photoconductive support surface and, then, causing the toner mixture to be attracted onto the electrostatic latent image on the photoconductive support surface. In this process, for transporting particles of the toner mixture from the hopper towards the photoconductive support surface, the electrostatic force of attraction exerted between the toner of low volume resistivity and that of high volume resistivity must be higher than the magnetic force of attraction exerted by the magnet housed inside the sleeve or shell, or otherwise the electrostatic force of attraction necessary to bind the toners of low and high volume resistivities together may be overcome by the magnetic force of attraction of the magnet, resulting in separation of the toners of low and high volume resistivities from each other, the consequence of which is that both the toner of low volume resistivity and that of high volume resistivity will not uniformly be applied onto the electrostatic latent image on the photoconductive support surface.
On the other hand, the developing material disclosed in the above mentioned copending application is comprised of a mixture of a magnetic toner of a volume resistivity within the range of 10.sup.5 to 10.sup.14 .OMEGA..multidot.cm and a non-magnetic and electrically insulating toner and is used in the electrophotographic copying method using the magnetic brush development technique. During the development of the toner image, the non-magnetic and electrically insulating toner particles are attracted onto the electric charge pattern on the photoconductive support surface by the effect of the electric charge opposite in polarity to that of the pattern on the photoconductive support surface, which has been charged as a result of frictional electricity, whereas the magnetic toner particles behave in a manner similar to the one-component developing material. Furthermore, during the transfer of the toner image from the photoconductive support surface to the sheet of final support material, both the non-magnetic and electrically insulating toner and the magnetic toner are transferred by the effect of an electrical mirror image force and van der Waal's forces.
The developing material of the composition disclosed in any one of the above mentioned publication and copending application is, because of the absence of unconsumable carrier, free from such problems, e.g., deterioration of carrier beads and replenishment of the toner particles, as involved in the two-component developing material, i.e., the toner-carrier mixture, and is, unlike the one-component developing material of the composition hereinbefore described, useable in transferring the toner image from the photoconductive support surface to a sheet of final support material by the effect of a corona discharge that charges the sheet of final support material.
However, while conducting a series of experiments using a the developing material of the composition disclosed and claimed in the above mentioned copending application, the present inventors have found that the developing material itself, or the practical use of thereof, involves the following problems left unsolved.
(1) When the magnetic toner of a relatively low range of volume resistivity, 30.mu. in average particle size, was mixed with the non-magnetic and insulating toner of 15.mu. in average particle size in a proportion of 9:1 and the resultant mixture was used in developing the toner image on the photoconductive support surface while the bias voltage and the magnetic attractive force of the magnet were so adjusted that no background deposition could occur, the resultant image reproduced on the sheet of final support material showed an acceptable contrast between the toner deposition and the background, but an insufficient resolution. On the other hand, when the magnetic toner of a relatively high range of volume resistivity, 30.mu. in average particle size, was mixed with the non-magnetic and insulating toner of 15.mu. in average particle size in a proportion of 9:1 and the resultant mixture was used in developing the toner image on the photoconductive support surface while the bias voltage and the magnetic attractive force of the magnet were so adjusted that no background deposition occurred, the resultant image reproduced on the sheet of final support material showed the reverse effect, that is, an acceptable resolution, but a low contrast.
In this way, the use of the developing material of the composition disclosed and claimed in the above mentioned copending application does not result in the high fidelity reproduction of the image of both high resolution and high contrast.
(2) The electrophotographically reproduced image of pale characters and/or fine lines often showed the reduced line width with reduced contrast.
(3) When the magnetic brush development was effected during the electrophotographic reproduction of, for example, an area image or a consecutive image while the magnet housed inside the sheeve or shell was rotated at a rate of 1,000 rpm as reduced from 2,000 rpm for the purpose of avoiding the possible heating under the influence of an eddy current, the reproduced image on the sheet of final support material was such that the contrast between the toner deposition and the background was gradually reduced from the front of the image towards the rear of the same, thereby lacking a high fidelity reproduction capability.
These problems must be solved by all means to enable the developing material to be commercial.